The molecular mechanisms by which C. parvum interacts with host cells and causes illness are unknown. This investigator's laboratory has partially isolated the first C. parvum specific aminopeptidase that is present on the membrane of the invasive sporozoite. The goals of this study are to characterize the biochemical properties and human immune response to the aminopeptidase of C. parvum sporozoites, and to define its role in the pathogenesis of infection.